There have recently appeared many portable electronic devices such as cell phones, mobile computers and electric tools, and thus, secondary batteries with a high voltage and a high energy density as a power source for such devices have been regarded as important. Furthermore, from the environmental point of view, shift to vehicles utilizing clean energy such as hybrid cars, fuel-cell cars and electric cars has begun and absolutely needs developing a highly durable secondary battery used for their power supply unit. Among secondary batteries used for these applications, a lithium ion secondary battery has gathered attention with an expectation that it could achieve a high energy density, and a number of studies have been made attempting to provide a battery with a higher capacity and a longer life.
A lithium ion secondary battery generally has a laminate structure comprised of a cathode in which a cathode active material layer containing a cathode active material and a binder is formed on a collector; an anode in which an anode active material layer containing an anode active material and a binder is formed on a collector; and a separator intervening between the cathode and the anode disposed in such a manner that their active material layers face each other. Lithium ion secondary batteries have only one laminate structure described above as a whole or two or more such laminates. The interior of the battery is filled with electrolyte composition containing a lithium-containing electrolyte salt and a nonaqueous solvent.
A cathode active material is predominantly a lithium-containing metal compound such as lithium-cobalt complex oxide and lithium iron phosphate. An anode active material is predominantly a carbon material such as graphite having a multilayer structure where lithium ions can be inserted/removed between layers. A cathode and anode can be produced by kneading the above active material, a binder, a solvent and, if necessary, an additive such as an electric conduction aid to prepare an active material slurry, applying the slurry to a collector and removing the solvent by drying to form an active material layer. Furthermore, it is, if necessary, pressed by a rolling press machine or the like.
Examples of a nonaqueous solvent used for an electrolyte composition include carbonate ester compounds such as ethylene carbonate (EC) and propylene carbonate (PC). In general, an electrolyte composition containing EC is widely used, but PC has a lower melting point than EC, having an advantage that it is useful for the used in a low temperature environment.
Here, particularly, a lithium ion secondary battery using graphite as an anode active material has a problem that during initial charging, a carbonate ester compound contained in an electrolyte composition reacts with graphite, leading to decomposition or deterioration. This reaction causes generation of a large amount of gas and formation of a passivation film on an electrode surface and thus generation of irreversible capacity, leading to problems such as reduction of initial charge/discharge efficiency, deterioration of cycle characteristics and safety deterioration of a battery. Furthermore, for an electrolyte composition containing PC, when lithium ions are inserted between graphite layers, co-insertion of solvated PC occurs, causing decomposition of PC and furthermore deterioration of graphite, which leads to capacity decrease of a secondary battery.
In terms of the above problems, it is known that decomposition and deterioration reaction can be inhibited using a polymer having an oxygen-containing functional group capable of coordinating to a lithium ion in a molecular structure such as polyvinyl alcohol, polymethacrylic acid and polyacrylic acid as a binder (see Patent Reference Nos. 1 to 3). It is believed that by coating graphite with a polymer having an oxygen-containing functional group capable of coordinating to a lithium ion in a molecular structure, desolvation of solvent molecules from lithium ions solvated with EC or PC can be accelerated, contact of graphite with solvent molecules can be prevented, and co-insertion of solvent molecules between graphite layers can be prevented.
Patent Reference No. 1 describes a battery electrode using polymethacrylic acid as a binder. However, polymethacrylic acid which has low solubility in water needs the use of an organic solvent as a solvent for preparing an active material slurry containing an active material and a binder. Therefore, in the light of improving workability, a binder with which an active material slurry can be prepared without using an organic solvent is needed.
Patent Reference Nos. 2 and 3 have described an active material slurry using polyvinyl alcohol or a polyacrylic acid salt as a binder and water as a solvent. The use of polyvinyl alcohol as a binder leads to a problem of increase of electrode polarization. Large electrode polarization leads to a large electrode resistance during charge/discharge, which prevents smooth insertion/removal of lithium, resulting in deterioration of charge/discharge properties.
When an acrylic polymer is used as a binder, an active material slurry frequently becomes too viscous to form a good active material layer. Furthermore, an acrylic polymer generally has a high glass-transition point (Tg), so that it frequently forms a hard and brittle active material layer, which causes cracks during production of a battery, leading to tendency to deterioration in productivity.